Atheism: The rejection of belief in any divine or supernatural entities. In the current vernacular, this relates especially to deities that have a personal relationship with followers of these divine entity or entities--the very crux of theism. Atheists, it should be noted, do not deny the possibility of god or primal source. However, not only is there zero evidence to support such a claim, there is overwhelming scientific evidence to reject it. The primal source claim, for example, leads to various ontological issues whereby logic breaks down and faith inevitably takes over (who created the creator? Etc.). Atheists see no reason to fill these empty holes with steadfast belief grounded in assumption. For this reason, they reject belief in god until good evidence is presented.
Agnosticism: This is simply not claiming to know whether a deity or deities exist. An Agnostic can still be a believer in a deity, but would only admit that such a belief is unprovable. An agnostic can also be an atheist, which would look similar to the definition for atheism with the exception of the Agnostic CHOSING to not believe based on whatever reasons he or she has for said disbelief. The atheist, by contrast, CANNOT believe god exists because of the lack of evidence--the scientific method makes it necessary to reject god like any other untestable fantastic claims. Atheist=bound to the evidence. Agnostic=more willing to take a leap of faith and willing to excuse lack of evidence due to (fill in the blank).
Deism: Belief in a divine creator, but either hard skepticism or outright disbelief that this creator has or ever had any interaction with our world by way of miracles or supernatural events. See the Jefferson Bible, or Spinoza's work.